1. Field of the Invention
This invention relates to a new silicotitanate molecular sieve ion exchange material for the capture and immobilization of divalent cations from aqueous and/or hydrocarbon solutions, including elements such as radioactive strontium or industrial RCRA metal cations. The invention further relates to the ability to either recycle the captured metal for future use or to encapsulate the cation through thermal treatment of the molecular sieve to a condensed phase.
2. Description of the Prior Art
It is known that crystalline silicotitanates (CST; commercially available through UOP LLP, as IE-911) can selectively remove 100 ppm Cs and Sr cations from 5M Na+ solutions over a broad pH range (1 to 14) and, because of this, they have been found effective to clean up radioactive 137Cs and 90Sr from waste tanks at the US Department of Energy Hanford waste tanks. To immobilize the Cs-loaded CSTs, there is used a combination of high activity waste and melting with borosilicate glass to create a glass log waste form which can be stored indefinitely. However, since titania induces crystallization and separation in borosilicate glass, the Cs-CST must be diluted to a few weight percent, thus increasing the volume and the cost of waste form production. It has already been shown by Y. Su, [Y. Su, et. al, MRS Conference Proceedings (Boston) xe2x80x9cEvaluation of Cesium Silicotitanates as an Alternative Waste Formxe2x80x9d p. 457 (1997)]. that a waste form that is more chemically durable than borosilicate glass logs can be generated by direct thermal conversion of Cs-CST. However, an improvement in the isolation of divalent radioactive and industrial waste remains quite desirable.
Therefore, an object of this invention is to provide a new type of inorganic molecular sieve materials. Another object is to provide inorganic materials that are mechanically and chemically stable and are free from the traditional problems associated with organic-based molecular sieves. A further object is to provide microporous compounds that can be used for radionuclides and industrial metals sorption. A still further object is to provide a molecular sieve that can be back-exchanged by acid wash to recover/recycle the sequestered metal cation. Still another object is to provide molecular sieves that can be thermally condensed to form leach resistant phases for radionuclide storage.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description, or will be learned by practice of the invention.
There is now disclosed that the radionuclides 137cesium, and to a lesser extent 90strontium, as well as divalent cations of several metals can be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger, namely a Csxe2x80x94Sixe2x80x94Tixe2x80x94O phase. However, an improved divalent ion exchanger and its condensed counterpart phase have now been hydrothermally synthesized, characterized, evaluated, and described in this specification. The viability of the new materials for divalent sequestration and subsequent encapsulation is based on chemical, mechanical, and thermal stability, leachability, and ion exchange capabilities. The two novel Csxe2x80x94Sixe2x80x94Tixe2x80x94O phases are Cs3TiSi3O9.5.3H2O (SNL-B), a porous phase which adsorbs the divalent cation metal, and Cs2TiSi6O15 (SNL-A), a condensed stable form in which the metal is immobilized for storage purposes, if that is desired. The two phases are also identified by their crystallographic parameters: SNL-B, orthorhombic, unit cell parameters a=10.83 xc3x85, b=7.43 xc3x85 and c=7.11 xc3x85; SNL-A, monoclinic, Cc space group, unit cell parameters: a=12.998 (2) xc3x85, b=7.5014 (3) xc3x85, c=15.156 (3) xc3x85, xcex2=105.80 (3)xc2x0.